Combine harvesters are provided with cleaning systems used to remove chaff and other residue from the threshed grain. Within the cleaning system, oscillating sieve assemblies in conjunction with air flow remove the chaff from the threshed grain, the latter falling through the chaffer and sieve assembly to an oscillating clean grain pan. The clean grain pan, in turn, directs the clean grain to a discharge auger that elevates the grain to an onboard storage bin. A second oscillating pan directs materials other than grain over the edge of the bottom sieve assembly to a different discharge outlet for recirculation back through the threshing, separating and cleaning apparatus to extract the previously unthreshed grain. A fan produces an airstream through the chaffer and sieve assembly that entrains the lighter non-grain particles and carries them out the rear of the harvester. The fan draws air from the outside through one or more air intakes, which may lead to crop residue being drawn toward the air intakes as well. For instance, for some crops, such as corn, and under certain conditions, corn leaves may adhere to leaf screens located on the bottom of the combine harvester, resulting in the leaf screen becoming plugged and hampering the cleaning process.